Transistor circuit for producing current pulses through a variable impedance



Dec. 29, 1964 'r. J. TULP ETAL 3,163,774

TRANSISTOR cmcurr FOR PRODUCING CURRENT PULSES THROUGH A VARIABLE IMPEDANCE Original Filed July 22. 1957 INVENTOR. THEODORUS J. TULP HEINE AR. DE MIRANDA Y B WK" AG NT United States Patent 9 rnarssrsron cracpri roa rnonncrnn Conan-Na" rutsns reaction A yam ABLE nurnomscn Theodorus Joannes Tulip Heine Andries Rodrigues de Miranda, Eindhoven, Netherlands, assignors to North American Fhilips Company, inc, New York, N.Y., a corporation of Elicia; tu'e Original application .iuly 22, 1957, Ser. No. 673,365, now Patent No. 2,956,174, dated Oct. 11, 196i). Divided and this application Aug. 5, 196%, Ser. No. eases 4 Claims. (Cl. 37-8.5)

The present application is a division of U.S. patent application Serial No. 673,365, filed July 22, 1957, now US. Patent 2,956,174.

This invention relates to transistor circuits for producing current pulses of substantially constant amplitude through a variable load impedance, for example through a series of ferromagnetic memory elements, control pulses being supplied to the base of the transistor and the load impedance being included in its collector-emitter circuit in series with a source of collector voltage.

, In computers and in other similar applications of the pulse technique, it is frequently desirable for current pulses of substantially constant amplitude to be passed through an impedance. The impedance, however, frequently varies with operating conditions. In orderto maintain the amplitude of the current pulses substantially constant, the current source must have a comparatively high natural impedance. In such applications use is increasingly made of transistors, since they have important advantages with respect to other pulse sources; they occupy very little space, the required supply voltage is comparatively low and the efliciency of a transistor operating as a switch is much higher than that, for example, of a tube, so that very little energy is unnecessarily converted into heat. On the other hand, very small cores of ferromagnetic material having a substantially rectangular hysteresis loop are increasingly being used as memory elements, more particularly in computers and other analogous apparatus. The combination of small ferromagnetic cores as memory elements and of transistors as reading-out and/ or control elements is very attractive. However it is usually necessary for a whole series of memory elements to be controllable by means of the same s urce of pulses. According to whether a memory core was or was not magnetized in the reverse direction prior to control, it does or does not produce a counterelectromotive force during control, so that the effective load impedance connected to a source of current pulses controlling a series of such cores varies in accordance with the magnetic state of these cores. If, for example, all of the cores are premagnetized in the reverse direction the value of the load impedance is maximum, whilst it is much smaller if only one or two cores of a series of,

for example, 40 cores are premagnetized in said reverse direction.

A very simple transistor circuit for. controlling a series 1 current produced by the control pulses in the base emitter circuit of transistor 1 is limited by this resistor.

To control any arbitrary core of the'series of memory The series of ferroelements 5 used in the circuit shown in FIG. 1, a minimum current I is required (see FIG. 2). Assuming that the transistor 1 can pass twice as large a current I and that the current passed is to be prevented from unduly varying in accordance with the magnetic state of the cores and thus approaching the minimum value l the value for the total resistance in the circuit :of the current source 4 must be chosen to be comparatively high. The voltage I" this current source then becomes correspondingly high and readily exceeds the maximum collector-emitter voltage permissible for the transistor. In this case, it is assumed that the transistor 3 operates below the curvature tively for a given amplitude of the current pulse, whilst.

line B represents a load characteristic corresponding to the maximum values permissible for the collector current and the collector voltage.

If a magnetization inverting current pulse having an intensity I greater than I is supplied to a Winding of a- V premagnetized magnetic core having a rectangular hysteresis loop, so that the number of ampere turns required for changing-over the magnetization of the core is reached or exceeded, the magnetization of the core changes-over collector current 1 at a speed which increases in proportion to I-I A reading pulse produced due to change-over of the magnetiza-.

' tion of the core has a corresponding length T. The core is characterized inter alia by a given A man, that is to ay by the total variation in flux between the two opposite states of saturation, and it has been found that the integral of the instantaneous value V of the amplitude of the reading pul e is constant over the pulse length T and proportioual to A max. it the value of I-I is comparatively small, the magnetization of the core changes over only very slowly, producing a weak read-out pulse of small amplitude. When using such cores, it is therefore necessary to utilize control pulses of an intensity such that ZI does not fall below a given minimum value. Furthermore, when controlling by means of a transistor, the value of} is limited by the maximum permissible From this it is apparent that it is necessary for the spread A1 of. the pulse currents to be limited to a minimumyalue. With the load characteristic B, 1 (by change-over or" the magnetization .in only The spread Al is thus considerable and approximately Of I 'I9. V By. increasing the voltage of the pulse source and of the resistor connected in series with the memory elements,

a load characteristic B (FIG. 2) would be obtained, if

.. the value of the maximum current} remainsconstant.

The spread I I is thus reduced to about'45% of l -I the collector voltage during the pulses beiug'at most equal to Y FIG. 2 shows a third load characteristic B, which for V passes through a point corresponding to. a-current i smaller than I the maximum permissible collector voltage not being exceeded during of I l or. less than 20% of 1 7-1 However, there eneawa arises the difiiculty that, with the load characteristic B or B", the voltage required to pass a current greater than, or equal to I through the resistance of the whole load circuit of the current source is several times higher than the maximumcollector voltage ,V permissible for the transistor.

The object of the invention is to overcome said dithculty. The transistor circuit according to the invention is characterized in that a controllable impedance comprising an inductance is connected in series with the load impedance, said inductance being coupled to such a control circuit that the impedance exhibits a comparatively low value below a predetermined collector-current and i efiec-t, several embodiments will now be described more fully, by way of example, with reference to the accompanying drawings, in which:

FIGS. 3 and 4 show the diagrams of two diiierent embodiments. 7

The embodiment shown in FIG-3 is very similar to the circuit shown in FIG. 1. However, it comprises a controllable impedance constituted by an inductance 9' for example of 300 1th., in series with a second voltage source 16', of comparatively low' voltage, a rectifier 11, and a resistor 12. The voltage source 19' in series with the inductance 9' is included in the reverse direction in the collector path of transistor 1, the rectifier 1 1 in series with the resistor 12 being connected in parallel with the series-combination of the inductance 9' and the source 10 in the forward direction with respect to this voltage.

source.

In the rest condition, a current flows from the source 16 through the inductance 9, the rectifier 11 and the resistor 12. The resistor 12 limits this current to the desired value which is related to the peak value of the current pulses, which can be passed through'the load impedance 5. The transistor 1 is cut oh and the voltage applied between its emitter and base and its collector is substantially equal to that of the direct-current source 4 plus that of the source 16. When a current pulse is supplied to the input winding 2, the emitter-collector path of the transistor 1 is suddenly opened, resulting in a strong decrease of the voltage across rectifier 11, so that this rectifier is cut off. The current which flowed through the rectifier now passes through the load impedance 5 and the transistor 1 and, since this current cannot rapidly change in value due to the eflfect of the inductance 9, the initial value of the current pulse through the load impedance 5 is substantially equal to the value of the current which flows through the inductance 9', the rectifier 11 and the justed value at the moment when thesecond control pulse Consequently, the initial.

111. ance of the inductance 9' are chosen to be such that the current through the load impedance 5 increases after reaching an initial value substantially equal to the adjusted value of the current through the inductance 9'. However, up to the end of the control pulse, this increase must be limited to, for example, 10%, since otherwise a spread in the form of the read-out pulses would result. Consequently, the time-constant L/R is preferably several times greater than the length of the control pulses.

The transistor of the circuit shown in FIG. 3 actually operates as a switch which changes-over the current through the inductance 9' during the current pulses across the load impedance, the rectifier 11 then being cut-off due to the fact that the potential at the common point of the rectifier 11 and the inductance 9' becoming less than that at the common point of the voltage sources 4 and 10.

In the second embodiment shown in FIG. 4, the controllable impedance is constituted by an inductance 13 which is connected in series with the load impedance 5. This inductance is arranged on a ferromagnetic core 14, which is pre-magnetized in a manner such that the inductance is saturated and has a low impedance. This pre-magnetization is brought about by means of a second winding 15 arranged on the core 14 and connected to a direct-current source. In FIG. 4, the supply voltage source -i for the emitter-collector circuit of transistor 1 is also used for the premagnetization of core 14. The winding 15 is connected to the common point of the source and of the load circuit 5,13 and also to the emitter of the transistor 1 via a resistor 16. If a control pulse is supplied to the input winding 2, a current pulse flows through the load impedance 5 and the winding 13. The respective directions of the windings 13 and 15 are chosen to be such that this current pulse suppresses at least partially the pre-rnagnetizationof core 14. This results in the impedance of winding 13 being greatly increased, so that the'amplitude of the current pulses through this winding and through the load impedance 5 is limited by the impedance of this winding and is sub.- stantially independent of the value of the load impedance. The ratio between the number of turns n of the winding 13 and the number of turns :1 of the winding 15,

the voltage of the pre-magnetization current source and the value of resistor 16' are chosen to be such that the current through the winding 15 saturates the core 14 when the transistor is cut Oh, but is less than li r 7L2 wherein a is the base-collector current amplification factor of the transistor 1 and i is the current produced by the current pulse in the base-emitter circuit of this transistor. a

At the end, of each current pulse through the load impedance 5 and the winding 13, a current pulse counteracting the pro-magnetizationcurrent is induced through the winding 15; If this current flows through resistor 16, it is limited by it and the energy accumulated inthe inductance 13 and its core 14 is partly dissipated in resistor 16 and also produces at the terminals of the In order to suppress 'this counter-voltage surge, a rectifier 17 is connected in of the sources 4 and 10",the minimum value of the total resistance R of the emitter-collector circuit of transistor 1 in the conductive state and the natural resistparallel with resistor 16 and connected in the cut-otf directionwithrcspect to the voltage source 4, so that the pro-magnetization current is determined'by the resistor 1 3. respect to the current pulse induced through the winding 15 at the. end of each current pulse so that said'current pulse is fed back to the voltagesource 4 via rectifier 17. If this voltage source is, for example a battery, it is,

recharged by-thecurrent pulse thus fed back, sovthat very little energy is unnecessarily dissipated in the windings 13, 15 and in the core .14.' i

However, the rectifier 17 is conducting with What is claimed is:

l. A transistor circuit for producing current pulses of substantially constant amplitude through a variable load impedance, comprising a transistor having base, emitter and collector electrodes, means for applying control pulses to said base electrode, a source of collector voltage, a variable load impedance connected in the collector-emitter circuit of the transistor in series with said source of collector voltage, a controllable impedance comprising an inductance connected in series with said lead impedance, said inductance being coupled to a control circuit comprising a rectifier shunting said inductance and at least a portion of said collector voltage source, the polarity of said rectifier being in the forward direction with respect to said portion of the voltage source, whereby a current flows through said rectifier and said inductance when the transistor is nonconducting, the amplitude of said current being substantially equal to the desired a constant value of the amplitude of the current pulses.

2. A circuit as claimed in claim 1, wherein the time constant of the circuit constituted by said inductance and said rectifier is substantially smaller than the minimum time interval between two successive control pulses, whereby the amplitude of the current through said circuit is substantially independent of said time interval 3. A circuit as claimed in claim 1, wherein the time constant of the circuit constituted by said inductance, said load impedance and the collector-emitter path of said transistor in the conducting state is substantially greater than the lengthof the applied control pulses,

4. A circuit as claimed in claim 2, wherein the time constant of the circuit constituted by said inductance, said load impedance and the collector-emitter path of said transistor in the conducting state is substantially greater than the length oftthe applied control pulses.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A TRANSISTOR CIRCUIT FOR PRODUCING CURRENT PULSES OF SUBSTANTIALLY CONSTANT AMPLITUDE THROUGH A VARIABLE LOAD IMPEDANCE, COMPRISING A TRANSISTOR HAVING BASE, EMITTER AND COLLECTOR ELECTRODES, MEANS FOR APPLYING CONTROL PULSES TO SAID BASE ELECTRODE, A SOURCE OF COLLECTOR VOLTAGE, A VARIABLE LOAD IMPEDANCE CONNECTED IN THE COLLECTOR-EMITTER CIRCUIT OF THE TRANSISTOR IN SERIES WITH SAID SOURCE OF COLLECTOR VOLTAGE, A CONTROLLABLE IMPEDANCE COMPRISING AN INDUCTANCE CONNECTED IN SERIES WITH SAID LOAD IMPEDANCE, SAID INDUCTANCE BEING COUPLED TO A CONTROL CIRCUIT 